As described by F. Capasso et al. in Solid State Communications, Vol. 102, No. 2-3, pp. 231-236 (1997) and by J. Faist et al. in Science, Vol. 264, pp. 553-556 (1994), which are incorporated herein by reference, a QC laser is based on intersubband transitions between excited states of coupled quantum wells and on resonant tunneling as the pumping mechanism. Unlike all other semiconductor lasers (e.g., diode lasers), the wavelength of the lasing emission of a QC laser is essentially determined by quantum confinement, i.e., by the thickness of the layers of the active region rather than by the bandgap of the active region material. As such it can be tailored over a very wide range using the same semiconductor material. For example, QC lasers with InAlAs/InGaAs active regions have operated at mid-infrared wavelengths in the 3.5 to 13 .mu.m range. In diode lasers, on the contrary, the bandgap of the active region is the main factor in determining the lasing wavelength. Thus, to obtain lasing operation at comparable infrared wavelengths the prior art has largely resorted to the more temperature sensitive and more difficult-to-process lead salt materials system.
More specifically, diode lasers, including quantum well lasers, rely on transitions between energy bands in which conduction band electrons and valence band holes, injected into the active region through a forward-biased p-n junction, radiatively recombine across the bandgap. Thus, as noted above, the bandgap essentially determines the lasing wavelength. In contrast, the QC laser relies on only one type of carrier, i.e., it is a unipolar semiconductor laser in which electronic transitions between conduction band states arise from size quantization in the active region heterostructure.
In earlier QC lasers both the waveguide core (including the active region) and the waveguide cladding comprised ternary Group III-V compounds such as InAlAs and InGaAs grown by molecular beam epitaxy (MBE). In order to enhance heat removal during lasing operation, InP has been substituted for the InAlAs cladding material. See, J. Faist et al., Appl. However, these QC lasers were etched in the form of a mesa stripe in which the side walls of the active InAlAs/InGaAs region were usually covered by an insulating dielectric material (such as silicon nitride or silicon dioxide) with an overlaid top-contact metalization (which introduces loss). Neither of these materials is suitable for heat transport or for optical mode confinement. As a consequence, such designs tend to have threshold current densities that limit the optical output power that can be attained without damaging the laser.
Thus, a need remains in the QC laser art for a laser design that addresses the heat transport and mode confinement problem in a way that lowers the threshold current density and enables higher output power operation.